Internet Engineering Task Force (IETF) J. Reschke
Request for Comments: 8187 greenbytes
Obsoletes: 5987 September 2017
Category: Standards Track
ISSN: 2070-1721
Indicating Character Encoding and Language for HTTP Header FieldParameters
Abstract
By default, header field values in Hypertext Transfer Protocol (HTTP)
messages cannot easily carry characters outside the US-ASCII coded
character set. RFC 2231 defines an encoding mechanism for use in
parameters inside Multipurpose Internet Mail Extensions (MIME) header
field values. This document specifies an encoding suitable for use
in HTTP header fields that is compatible with a simplified profile of
the encoding defined in RFC 2231.
This document obsoletes RFC 5987.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8187.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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On the other hand, RFC 2231 defines an encoding mechanism for
parameters inside MIME header fields ([RFC2231]), which, as opposed
to HTTP messages, do need to be sent over non-binary transports.
This document specifies an encoding suitable for use in HTTP header
fields that is compatible with a simplified profile of the encoding
defined in RFC 2231. It can be applied to any HTTP header field that
uses the common "parameter" ("name=value") syntax.
This document obsoletes [RFC5987] and moves it to "Historic" status;
the changes are summarized in Appendix A.
Note: In the remainder of this document, RFC 2231 is only
referenced for the purpose of explaining the choice of features
that were adopted; therefore, they are purely informative.
Note: This encoding does not apply to message payloads transmitted
over HTTP, such as when using the media type "multipart/form-data"
([RFC7578]).
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
This specification uses the ABNF notation defined in [RFC5234]. The
following core rules are included by reference, as defined in
[RFC5234], Appendix B.1: ALPHA (letters), DIGIT (decimal 0-9), HEXDIG
(hexadecimal 0-9/A-F/a-f), and LWSP (linear whitespace).
This specification uses terminology defined in [RFC6365], namely:
"character encoding scheme" (abbreviated to "character encoding"
below), "charset", and "coded character set".
Note that this differs from RFC 2231, which uses the term "character
set" for "character encoding scheme".
3. Comparison to RFC 2231 and Definition of the EncodingRFC 2231 defines several extensions to MIME. The sections below
discuss if and how they apply to HTTP header fields.
In short:
o Parameter Continuations aren't needed (Section 3.1),
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o Character Encoding and Language Information are useful, therefore
a simple subset is specified (Section 3.2), and
o Language Specifications in Encoded Words aren't needed
(Section 3.3).
3.1. Parameter ContinuationsSection 3 of [RFC2231] defines a mechanism that deals with the length
limitations that apply to MIME headers. These limitations do not
apply to HTTP ([RFC7231], Appendix A.6).
Thus, parameter continuations are not part of the encoding defined by
this specification.
3.2. Parameter Value Character Encoding and Language InformationSection 4 of [RFC2231] specifies how to embed language information
into parameter values and also how to encode non-ASCII characters,
dealing with restrictions both in MIME and HTTP header field
parameters.
However, RFC 2231 does not specify a mandatory-to-implement character
encoding, making it hard for senders to decide which encoding to use.
Thus, recipients implementing this specification MUST support the
"UTF-8" character encoding [RFC3629].
Furthermore, RFC 2231 allows the character encoding information to be
left out. The encoding defined by this specification does not allow
that.
3.2.1. Definition
The presence of extended parameter values is usually indicated by a
parameter name ending in an asterisk character. However, note that
this is just a convention, and that the extended parameter values
need to be explicitly specified in the definition of the header field
using this extension (see Section 4).
The ABNF for extended parameter values is specified below:
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Producers MUST use the "UTF-8" ([RFC3629]) character encoding.
Extension character encodings (mime-charset) are reserved for future
use.
Note: Recipients should be prepared to handle encoding errors,
such as malformed or incomplete percent escape sequences, or
non-decodable octet sequences, in a robust manner. This
specification does not mandate any specific behavior; for
instance, the following strategies are all acceptable:
* ignoring the parameter,
* stripping a non-decodable octet sequence, and
* substituting a non-decodable octet sequence by a replacement
character, such as the Unicode character U+FFFD (Replacement
Character).
3.2.2. Historical Notes
The RFC 7230 token production ([RFC7230], Section 3.2.6) differs from
the production used in RFC 2231 (imported from Section 5.1 of
[RFC2045]) in that curly braces (i.e., "{" and "}") are excluded.
Thus, these two characters are excluded from the attr-char production
as well.
The <mime-charset> ABNF defined here differs from the one in
Section 2.3 of [RFC2978] in that it does not allow the single quote
character (see also RFC Errata ID 1912 [Err1912]). In practice, no
character encoding names using that character have been registered at
the time of this writing.
For backwards compatibility with RFC 2231, the encoding defined by
this specification deviates from common parameter syntax in that the
quoted-string notation is not allowed. Implementations using generic
parser components might not be able to detect the use of quoted-
string notation and thus might accept that format, although invalid,
as well.
[RFC5987] did require support for ISO-8859-1 ([ISO-8859-1]), too; for
compatibility with legacy code, recipients are encouraged to support
this encoding as well.
3.2.3. Examples
Non-extended notation, using "token":
foo: bar; title=Economy
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Non-extended notation, using "quoted-string":
foo: bar; title="US-$ rates"
Extended notation, using the Unicode character U+00A3 ("£", POUND
SIGN):
foo: bar; title*=utf-8'en'%C2%A3%20rates
Note: The Unicode pound sign character U+00A3 was encoded into the
octet sequence C2 A3 using the UTF-8 character encoding, and then
percent-encoded. Also, note that the space character was encoded as
%20, as it is not contained in attr-char.
Extended notation, using the Unicode characters U+00A3 ("£", POUND
SIGN) and U+20AC ("€", EURO SIGN):
foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
Note: The Unicode pound sign character U+00A3 was encoded into the
octet sequence C2 A3 using the UTF-8 character encoding, and then
percent-encoded. Likewise, the Unicode euro sign character U+20AC
was encoded into the octet sequence E2 82 AC, and then percent-
encoded. Also note that HEXDIG allows both lowercase and uppercase
characters, so recipients must understand both, and that the language
information is optional, while the character encoding is not.
3.3. Language Specification in Encoded WordsSection 5 of [RFC2231] extends the encoding defined in [RFC2047] to
also support language specification in encoded words. RFC 2616, the
now-obsolete HTTP/1.1 specification, did refer to RFC 2047
([RFC2616], Section 2.2). However, it wasn't clear to which header
field it applied. Consequently, the current revision of the HTTP/1.1
specification has deprecated use of the encoding forms defined in RFC2047 (see Section 3.2.4 of [RFC7230]).
Thus, this specification does not include this feature.
4. Guidelines for Usage in HTTP Header Field Definitions
Specifications of HTTP header fields that use the extensions defined
in Section 3.2 ought to clearly state that. A simple way to achieve
this is to normatively reference this specification and to include
the ext-value production into the ABNF for specific header field
parameters.
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For instance:
foo = token ";" LWSP title-param
title-param = "title" LWSP "=" LWSP value
/ "title*" LWSP "=" LWSP ext-value
ext-value = <see RFC 8187, Section 3.2>
Note: The Parameter Value Continuation feature defined in
Section 3 of [RFC2231] makes it impossible to have multiple
instances of extended parameters with identical names, as the
processing of continuations would become ambiguous. Thus,
specifications using this extension are advised to disallow this
case for compatibility with RFC 2231.
Note: This specification does not automatically assign a new
interpretation to parameter names ending in an asterisk. As
pointed out above, it's up to the specification for the
non-extended parameter to "opt in" to the syntax defined here.
That being said, some existing implementations are known to
automatically switch to using this notation when a parameter name
ends with an asterisk; thus, using parameter names ending in an
asterisk for something else is likely to cause interoperability
problems.
4.1. When to Use the ExtensionSection 4.2 of [RFC2277] requires that protocol elements containing
human-readable text be able to carry language information. Thus, the
ext-value production ought to always be used when the parameter value
is of a textual nature and its language is known.
Furthermore, the extension ought to also be used whenever the
parameter value needs to carry characters not present in the US-ASCII
coded character set ([RFC0020]); note that it would be unacceptable
to define a new parameter that would be restricted to a subset of the
Unicode character set.
4.2. Error Handling
Header field specifications need to define whether multiple instances
of parameters with identical names are allowed and how they should be
processed. This specification suggests that a parameter using the
extended syntax takes precedence. This would allow producers to use
both formats without breaking recipients that do not understand the
extended syntax yet.
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Example:
foo: bar; title="EURO exchange rates";
title*=utf-8''%e2%82%ac%20exchange%20rates
In this case, the sender provides an ASCII version of the title for
legacy recipients, but also includes an internationalized version for
recipients understanding this specification -- the latter obviously
ought to prefer the new syntax over the old one.
5. Security Considerations
The format described in this document makes it possible to transport
non-ASCII characters, and thus enables character "spoofing" scenarios
in which a displayed value appears to be something other than it is.
Furthermore, there are known attack scenarios related to decoding
UTF-8.
See Section 10 of [RFC3629] for more information on both topics.
In addition, the extension specified in this document makes it
possible to transport multiple language variants for a single
parameter, and such use might allow spoofing attacks where different
language versions of the same parameter are not equivalent. Whether
this attack is effective as an attack depends on the parameter
specified.
6. IANA Considerations
This document does not require any IANA actions.
7. References7.1. Normative References
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
October 2000, <https://www.rfc-editor.org/info/rfc2978>.
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[XMLHttpRequest]
WhatWG, "XMLHttpRequest", <https://xhr.spec.whatwg.org/>.
Appendix A. Changes from RFC 5987
This section summarizes the changes compared to [RFC5987]:
o The document title was changed to "Indicating Character Encoding
and Language for HTTP Header Field Parameters".
o The introduction was rewritten to better explain the issues around
non-ASCII characters in field values.
o The requirement to support the "ISO-8859-1" encoding was removed.
o This document no longer attempts to redefine a generic "parameter"
ABNF (it turned out that there really isn't a generic definition
of parameters in HTTP; for instance, there are subtle differences
with respect to whitespace handling).
o A note about defects in error handling in current implementations
was removed, as it was no longer accurate.
Appendix B. Implementation Report
The encoding defined in this document is currently used in four
different HTTP header fields:
o "Authentication-Control", defined in [RFC8053],
o "Authorization" (as used in HTTP Digest Authentication, defined in
[RFC7616]),
o "Content-Disposition", defined in [RFC6266], and
o "Link", defined in [RFC5988].
As the encoding is a profile/clarification of the one defined in
[RFC2231] in 1997, many user agents already supported it for use in
"Content-Disposition" when [RFC5987] was published.
Since the publication of [RFC5987], three more popular desktop user
agents have added support for this encoding; see
<http://purl.org/NET/http/content-disposition-tests#encoding-2231-char> for details. At this time, the current versions of all
major desktop user agents support it.
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Note that the implementation in Internet Explorer 9 does not support
the ISO-8859-1 character encoding; this document revision
acknowledges that UTF-8 is sufficient for expressing all code points
and removes the requirement to support ISO-8859-1.
The "Link" header field, on the other hand, was more recently
specified in [RFC5988]. At the time of this writing, no user agent
except Firefox supported the "title*" parameter (starting with
release 15).
Section 3.4 of [RFC7616] defines the "username*" parameter for use in
HTTP Digest Authentication. At the time of writing, no user agent
implemented this extension.
Acknowledgements
Thanks to Martin Dürst and Frank Ellermann for help figuring out ABNF
details, to Graham Klyne and Alexey Melnikov for general review, to
Chris Newman for pointing out an RFC 2231 incompatibility, and to
Benjamin Carlyle, Roar Lauritzsen, Eric Lawrence, and James Manger
for implementers feedback.
Furthermore, thanks to the members of the IETF HTTP Working Group for
the feedback specific to this update of RFC 5987.
Author's Address
Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Münster, NW 48155
Germany
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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